Since oil was first discovered in 1859, the petroleum industry has expanded considerably. To keep pace with increasing worldwide needs of hydrocarbons, exploration has steadily been extended to all sedimentary basins capable of containing oil or natural gas. Considering the promising hydrocarbon potential of seabeds, the search for new fields has been extended to sedimentary basins at sea - initially right near the coast, then on continental shelves (at depths of less than 200 m of water), and now farther offshore at water depths of several thousand meters. Since 1960, this expansion of offshore activities has been based on the development of many original techniques to face up to the novelty and complexity of the problems to be solved. At the saure time, the development of offshore activities and, now, the conquest of difficult and deep offshore areas have led to the implementation of safety specifications and vert' strict regulations so as to guarantee effective protection of installations, persons and other users of the sea, and to avert any danger of possible pollution. If all current offshore activities are considered within the world energy context, the following two questions can be asked: (1) Is the cost of offshore marine operations becoming prohibitive? (2) Does the energy situation justify the conquest of deeper and deeper seas? It would be useless as of now to pretend to give a precise and categorical answer to all such questions. Nevertheless, an economic analysis of offshore activities, being cons-cious of passed experience, and the extrapolating of the trends observed already constitute extremely useful bases for making an assessment.